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Easy Laser Alarm

Leave No Entrance Unprotected

Description: Easy Laser Alarm
Assembly Time:1 hour depending on experience
Skill Level: Beginner
Designer: Mr Thomas

Protect your home and improve your electronics skills by building this Easy Laser Alarm kit. It helps you to protect your home using a 741 op-amp and 4017 decade counter as the brain and is powered by a 9V battery or if you prefer, connect a 9V wall transformer to the power connections in order to save your batteries.

Laser Alarm
Required tools and parts:

- A knife is helpful for the holes
- A drill to drill out holes in your project box
- Philips screwdriver for project box
- 9V battery
- Wire
- Hot glue
- Battery snap

Kit Includes:

LM741 Op Amp
4017 Decade Counter
Pushbutton Switch
Piezo Buzzer
Battery Holder
Laser Diode
1kΩ Resistor pack

Step 1 – Review components

First, check that you have all the necessary parts and tools. The only parts that are not included are a 9v battery and wire. Additional tools needed include a knife, a drill (for drilling holes in the project box), a screwdriver (Philips – for project box) and an optional hot glue gun (very helpful for keeping the circuit and photoresistor attached to the project box).

Step 2 – Understanding the circuit

The kit works by sensing the light from a laser beam hitting a photoresistor which lowers the resistance. Once the beam is broken the resistance increases and the 741 op-amp sends a pulse to the 4017 decade counter which sets off the buzzer. To reset the circuit the laser must be back on the photoresistor and the reset button must be activated. Multiple entrances can be protected with just one kit if mirrors are added to reflect the laser.


Step 3 – Build your circuit

To begin you will want to have your supplies ready. First push the two ICs into the breadboard in the center, making sure that the notch indicated by the "U" or "O" at the top of the IC is pointing up (the same direction) to avoid confusion while connecting the pins. Attach the positive power to pin 16 and the negative ground to pin 8. Use the circuit schematic to build the rest of your circuit, but don't attach the power leads and the button quite yet. For the included laser diode an arrangement of two AA or AAA batteries connected in series with a switch should power the 3V laser pointer module.

Build your circuit

Step 4 – Drill holes in your project box

Use a 7/16" drill bit to drill out a hole for the button. To make a small hole for the photoresistor, it helps to make a hole little smaller than the size of the component so it does not pick up light from anywhere but the laser beam. Finally, make an additional hole for your battery box leads.

Step 5 – Fitting the outside components

To begin, fit the pushbutton in its hole and secure it. Then attach two pieces of wire to the button. Next, thread the wires from the 9V battery box/wall transformer through the hole and into your project box. Finally, connect the wires to your protoboard. You can hot glue the photo-resistor to the project box to keep it in place as well as the power supply cables.

IMPORTANT - Test your circuit before using hot glue (optional, but recommended) to secure your project in its project box. Keep in mind that hot glue is hard to remove from components and wires.

Fitting the outside components

Step 6 – Completion and trial

Close your project box, put in a 9V battery and you're done! Shine the laser beam on the photoresistor and turn on the project. Break the beam and you should hear a loud beep. In order to reset, press the reset button while the laser is on the sensor.

To make your project more like a security system you can remove the 9V buzzer and use the circuit to trigger a more effective alarm system. To make the laser pointer more permanent, use a small project box and mount your laser pointer inside the box with a 4.5V-5V power supply to your laser pointer, or you can just use the batteries.

Warning! Working with electronics can be dangerous. Always use caution and follow all safety procedures. If you are uncertain of the dangers involved with a particular project be sure to seek assistance. Failure to follow safety procedures may result in injury or death.
If you have an electronics story you'd like to share, please send it to [email protected].